Process and composition for coating aluminum surfaces



nited States PatentO "lce PROCESS AND COMPOSITION FOR COATING ALUMINUM SURFACES Frank Palin Spruance, Jr., Ambler, and Nelson James Newhard, Jr., Oreland, Pa., assignors to Amchem Products, Inc., a corporation of Delaware No Drawing. Application April 3, 1952 Serial No. 280,364

2 Claims. (Cl. 1486.2)

This invention relates to the art of producing a chemically bonded chromate conversion coating on surfaces of aluminum or alloys thereof in which aluminum is the principal or predominant ingredient.

The principal object of the invention is to render aluminum surfaces highly resistant to corrosion regardless of whether or not the coated surface is subsequently given a finish of paint, lacquer, enamel, japan or the like.

Herefore, it has been known that the durability and corrosion resistance of aluminum surfaces even under abnormally severe corrosive conditions can be improved by forming thereon a chemically bonded chi-ornate conversion coating by subjecting them to the action of an aqueous acid solution of hexavalent chromium and fluorine-bearing compounds, such as hydrofluoric acid, hydrofiuosilicic acid, fluoboric acid and their soluble salts. Specific examples of such processes are described in United States Patents Nos. 2,276,353 and 2,507,956, and such processes are now very generally known to those skilled in this art as chromate conversion coverings.

The present invention is based upon the discovery that the corrosion resistance which can be imparted to surfaces of aluminum by treatments of the character just described can be still further enhanced if there be added to the baths which are employed in producing the coat-# ing ions of metal from the class consisting of zirconium, titanium and tin. Moreover, the use of such metal additions to coating baths of the character described makes it possible to operate the baths at considerably lower temperatures than has heretofore been customary. Indeed, with our invention, it is even possible to produce coatings at room temperature or lower. By room temperature is meant average living temperature, which is in the neighborhood of 72 F.

The form in which the dissolved zirconium, titanium or tin is introduced into the coating bath appears to make little or no difference so long as the composition of the solution is not otherwise adversely disturbed. For example, these metals should not be introduced along with reducing agents which would cause a loss of hexavalent chromium by the reduction thereof to trivalent chromium. We have found it very desirable to introduce the metals as fluozirconic acid, fluotitanic acid, fluostannic acid or soluble salts of such acids, etc. Indeed, as an actual matter of fact, their introduction as fluorine acids is highly desirable because when so introduced they serve as a source of fluorine.

There is no minimum limit for concentration of the metals in the operating bath because an improvement is achieved even when using only very small amounts. Furthermore, as far as we can ascertain, excess amounts appear to do no harm but, generally speaking, in the interest of economy, we prefer to use from 0.1 to 7.0 grams of the metal per liter of operating bath.

By way of specific example we cite the following:

2,851,385. Patented Sept. 9, 1958 2 Formula No. 1

G. Chromic d 1 6 Potassium zirconium fluoride 2.5 Ammonium borofluoridenr 7.6

Water to make 1 liter.

Other representative bath formulae which can be employed with our invention are as follows:

Formula N 0. 2

Chromic acid- 1 I a a.. v 8.4 Potassium zirconium fluoride 3.5 Boric acid 6.3 Ammonium bifluoride 4.0 Water to make 1 liter.

F orm ula No. 3

G. Chromic acid 8 Hydrofluoric acid 2.5 23% titanium trichloride 3.2 Water to make 1 liter.

4.5 ml. of 48% acid.

Formula N0. 4

Ammonium bifluoride 2.7 Chromic acid 6 Stannic chloride (SnC1 4.4 Water to make 1 liter.

Formula No. 5

G. Diammoniumsilicofluoride (NHqJgslFs 14 Chromic acid--- 10 Zirconium nitrate Zr(NO .5H O 12 Water to make 1 liter.

If desired, mixtures of the metals may be used. For instance, in Formula No. 4 as given above the amount of stannic chloride used may be reduced by means of additions of titanium trichloride or of zirconium nitrate or both.

In carrying out our improved process the surface to be coated should first be cleaned. The cleaning, which forms no part of the present invention, may be carried out by conventional methods. For instance, grease and dirt may be removed by dipping into a mild silicate alkali bath or by the use of an emulsion of a grease solvent. The clean work, either wet or dry, may be treated with a solution of the character described, such for example as the solutions of the formulae given above.

The treatment may be given by immersing the surface to be coated in the solution, by flowing or spraying the solution on the work or by any other convenient technique in which the solution is allowed to act upon the work. For example, if the solution of Formula No. 1 is used in a dip installation, it may be heated to approxi mately F. and the clean work immersed in it for approximately 30 seconds to 2 minutes. Upon rem-oval from the solution the article may be rinsed with clean water and dried and afterwards painted, if desired. Sometimes the rinsing may be done with a dilute solution of Formula No. 1, after which the article may be dried and then painted. Or, if the piece is not to be painted, it may simply be removed from the solution and dried.

As mentioned above, our invention makes it possible to operate with baths at considerably lower temperatures than has been possible heretofore. For instance, in prior practice coating baths of silico-fluoride and chromic acid have been used hot,fi. e., at or near the boiling point. With our solutions thisis not necessary, although some slight increase in temperature may be employed if desired because of the greater rapidity of the resulting actioiif Nevertheless, and by way 'of-specific example, Form'ulfNo. 5 as given above will produce excellent corrosion resistant coatings if the cleaned article is immersed in the bath for a period of 1 to 2 minutes at a temperature not exceeding 80 F. Subject to minor variations the formulas of the other examples will yield comparable results as to temperature and time of treatment needed to produce good coatings. This is a marked and very valuable advantage of our invention.

We claim:

1. In the art of producing a chromate conversion coating on surfaces of aluminum and alloys thereof in which aluminum is the predominant ingredient by treating the surface with an aqueous acid bath of the type which consists essentially of hexavalent chromium and fluorine in proportions capable of producing such a coating; the method which comprises including in the bath, as an addition agent, metal ions from the group consisting of zirconium, titanium and stannic tin in a quantity sufficient References Cited in the file of this patent UNITED STATES PATENTS 1,939,421 Tosterud Dec. 12, 1933 1,957,354 Prier May 1, 1934 2,276,353 Thompson Mar. 7, 1942 2,312,855 Thompson Mar. 2, 1943 2,348,698 Thompson May 9, 1944 2,357,219 Mott Aug. 29, 1944 FOREIGN PATENTS 546,466 Germany Mar. 12, 1932 

1. IN THE ART OF PRODUCING A CHROMATE CONVERSION COATING ON SURFACES OF ALUMINUM AND ALLOYS THEREOF IN WHICH ALIUMINUM IS THE PREDOMINANT INGREDIENT BY TREATING THE SURFACE WITH AN AQUEOUS ACID BATH OF THE TYPE WHICH CONSISTS ESSENTIALLY OF HEXAVALENT CHROMIUM AND FLUORINE IN PROPORTIONS CAPABLE OF PRODUCING SUCH A COATING; THE METHOD WHICH COMPRISES INCLUDING IN THE BATH, AS AN ADDITION AGENT, METAL IONS FROM THE GROUP CONSISTING OF ZIRCONIUM, TITANIUM AND STANNIC TIN IN A QUANTITY SUFFICIENT TO ENHANCE THE CORROSION RESISTANCE OF THE COATED SURFACE AND THEN TREATING THE SURFACE WITH SUCH BATH. 